230 SOME ASPECTS OF ECOLOGY 



survival at freezing temperatures. In order to survive it must 

 avoid ice-formation in its blood and tissues. Water loss by 

 concentrating salts and other solutes in the body-fluid lowers the 

 freezing point. The adsorption or " binding " of much of the 

 remaining water by the colloids of the body still further lowers 

 the freezing point. The relation of " bound " to free water appears 

 to be highest in those insects which are able to withstand the 

 greatest degree of cold. 



While certain insects, such as Rhodnius, are unaffected by 

 humidities ranging from to 90 per cent., even with twenty-four 

 hours' exposure, with others such as flea larvae the death point is 

 at a lower temperature in dry air. The cockroach, however, can 

 survive a higher temperature in dry air — 10° higher — than in moist 

 air (Bodenheimer, 1927). The adult Calliphora and the beetle 

 Ejpilachna, on the other hand, survive best at moderate humidities 

 of 60 per cent, or over. Davies (1928) has shown that atracheate 

 species of Collembola require a saturated atmosphere in order to 

 survive even a few hours at a constant temperature of 25°. On 

 the other hand, the tracheate form, Sminthurus viridis, proved 

 more resistant and was able to live fifteen hours at 20 per cent, 

 humidity before all were killed off. 



Experiments by Headlee (1917), on the effects of atmospheric 

 humidity on the bean weevil (Bruchus ohtectus) and the Angoumois 

 grain moth [Sitotroga cerealella), are noteworthy. The insects 

 were kept uniformly in the dark and at a constant temperature of 

 21-1°. In their late larval and pupal stages these two species 

 exhibited a marked reduction in rate of metabolism in relation 

 to increase of humidity. The pupal stage of the moth lasted 

 seventeen days in 100 per cent, humidity, and was reduced to 

 twelve days when the humidity was 21-8 per cent. ; in the case of 

 the weevil the corresponding periods were twenty-two days in 100 

 per cent, humidity, and fourteen days at 44-6 per cent, humidity. 

 With the adult insects the evidence pointed to decreased humidity 

 shortening the life of the moth, and lengthening that of the bean 

 weevil. In the case of the eggs of the latter insect, two days 

 longer were required for hatching when the humidity was 100 

 per cent., as compared with one of 23-6 per cent. Taking the life- 



